Cationic electrodeposition paint compositions excel in coating workability, and films formed thereof exhibit good corrosion resistance. Hence they are widely used as undercoat of electrically conductive metallic articles such as car bodies which require these properties.
Resins which are used in cationic electrodeposition paint compositions normally are produced using organic solvent, and are blended with the paint in the form of organic solvent-containing resin solutions (varnishes). Resulting cationic electrodeposition paint compositions, therefore, contain organic solvent.
Cationic electrodeposition paint compositions containing organic solvent show such technical effects as good hydrophilicity/hydrophobicity balance leading to high paint stability without degradation in water dispersibility of the resins which are the constituent component of the cationic electrodeposition paint compositions; good film thickness retention with time and finished appearance of the coating film; and excellent electrocoatability on galvanized alloy steel sheet. Heretofore such organic solvent as ethylene glycol monobutyl ether (boiling point, 171° C.), ethylene glycol monohexyl ether (boiling point, 208° C.), propylene glycol monopropyl ether (boiling point, 150° C.), cyclohexanone (boiling point, 145° C.) and the like have been used for cationic electrodeposition paint compositions.
Use of these organic solvents is restricted in recent years for consideration of environments, by volatile organic compound [hereafter may be referred to as (VOC)] regulation or harmful atmospheric pollutants (HAPs) regulation. Those organic solvents can be removed from cationic electrodeposition paint compositions by desolventing to provide low VOC paint compositions. However, when volatile organic compound (VOC) content of cationic electrodeposition paint compositions is reduced, such problems as deterioration in film thickness retention with time, finished appearance of coating film or electrocoatability on galvanized alloy steel sheet may occur.
Among proposals for solving the problems, for example, JP Sho 63 (1988)-92637A discloses electrodeposition paint containing cationic epoxy resin which is produced by adding organic acid and water to a resin obtained through reaction of (A) a composition comprising diglycidyl ether of at least one kind of polyol and diglycidyl ether of at least one kind of dihydric phenol, with (B) at least one kind of dihydric phenol, to convert the oxysilane groups to cationic groups. Coating film formed of the electrodeposition paint comprising the cationic epoxy resin, however, shows insufficient corrosion resistance.
JP 2003-221547A discloses cationic electrodeposition paint containing as the base resin xylene-formaldehyde resin-modified, amino group-containing epoxy resin formed through reaction of an epoxy resin having epoxy equivalent of 180-2,500 with xylene-formaldehyde resin and amino group-containing compound. The cationic electrodeposition paint containing the xylene-formaldehyde resin-modified, amino group-containing epoxy resin alone as the base resin, however, shows problems in film thickness retention when its VOC content is reduced. Furthermore, because the xylene-formaldehyde resin-modified, amino group-containing epoxy resin has insufficient water dispersibility, addition of a large amount of neutralizer is necessary to secure water dispersibility, which causes defects in finished appearance and electrocoatability on galvanized alloy steel sheet.
JP Hei 8 (1996)-245750A discloses cationic electrodeposition paint containing an epoxy resin (A) produced from diglycidyl ether of polyether polyol, which has an average epoxy equivalent of 350-5,000, or an epoxy resin (B) produced from diglycidyl ether of partially capped polyether polyol, which has an average epoxy equivalent of 300-5,000. However, the coating film formed of the cationic electrodeposition paint containing the resin as described in the patent publication has insufficient corrosion resistance.
JP 2001-3005A discloses that an electrodeposition paint containing less VOC and excelling in film-forming property, electrocoatability of galvanized alloy steel sheet and corrosion resistance can be obtained by blending alkylene-type polyether polyol such as polymethylene glycol, polyethylene glycol, polypropylene glycol or polybutylene glycol, or polyether polyol such as bisphenol alone or aromatic ring-containing polyether polyol obtained through reaction of bisphenol with glycol.
Also JP 2006-274234A discloses that an electrodeposition paint containing little volatile organic solvent (low VOC) which is free from pinhole occurrence when electrocoated on rust-preventive steel sheet and excels in corrosion resistance and paint stability can be obtained by blending specific polyether compound having a molecular weight not more than 1,000.
When a large amount of the polyether polyols as disclosed in JP 2001-3005A or the specific compound having a molecular weight not more than 1,000 as disclosed in JP 2006-274234A are added to electrodeposition paint, however, corrosion resistance of formed coating film may drop, or when mechanical load is exerted on the electrocoating bath over a prolonged period, there is a possibility that the paint stability may be reduced.